In carrying out the air hammer drilling of wells for various applications, the use is made of downhole air hammer apparatuses (air hammers) effecting rock breaking over the whole area of the well bottom, the cuttings (drillings) being removed to the surface with the exhaust air through the annulus between the well wall and the drill pipe string. The well head is provided with a dust trapping system which in the majority of applications proves inadequate and cannot ensure compliance with the sanitary rules governing dust lading at the drilling operator workplace.
The air hammers of the downhole type are of a very limited use in drilling geological exploration wells in alluvial mineral deposits in the permafrost areas because of thawing and collapse of well walls under the action of the exhaust air flow. Therefore, for the abovementioned reasons, the use of the downhole air hammers that do not have the internal return passages for drillings cannot bring a solution to the problem of obtaining a reliable information on the geological structure of a deposit (rock mass).
In order to improve reliability of the geological trial and to avoid thawing and collapse of well walls in drilling of permafrost rocks, the downhole air hammers are used in combination with a double drill pipe string, the well bottom cuttings being transported to the surface through the axial passage of the pipe string. However, in this case the arrangement of the bottom part of the drill string is much more complicated, and the return path for the exhaust cleaning agent enriched with drillings becomes rather intricate. The cleaning and energy carrier agent (compressed air) is first admitted from the intertube space of the double drill pipe string through passages of a distribution adapter to a centrally located air hammer. Then the cleaning agent washes and cleans the well bottom and enriched with drillings moves towards the well head between the well wall and the air hammer body to get into narrow return passages of the adapter and only after that it is directed into the inner pipe of the double pipe string. This return path of the air and drillings flow results in a high probability of the formation of drillings plugs in the passages of the adapter and glands in the well bottom zone and, in case of collapse of the well walls, circulation of the energy carrier can be interrupted and the drilling may stop.
These difficulties can be avoided to a large extent when a double drill pipe string is used with an annular air hammer having a central axial passage defining, together with the interior space of the string, a straight smooth-bore duct for drillings of a constant cross-section.
Known in the art is an annular air hammer apparatus (rock drill) (cf. FRG Pat. No. 2854461, Int. C1. E 21 C 3/24, 1978), comprising an annular rock breaking tool and an annular hammer regularly delivering blows to the tool, the hammer being accommodated in a cylindrical chamber having air distribution ports. The apparatus also has a reverse valve and an inner pipe for receiving drillings.
Such apparatuses are used for core drilling of wells with simultaneous action of the percussive force and tool rotation. They are made in the form of downhole air-powered devices, the compressed air being supplied through the space between the inner pipe string and the outer pipe string to the reverse valve wherefrom it is admitted to the air hammer apparatus. The exhaust of the energy carrier from the air hammer apparatus is effected through a space between the outer pipe and the rock breaking tool.
The prior art annular air hammer apparatuses feature a complicated structure of their air distribution devices imposing stringent requirements upon parameters of the energy carrier, as well as to the quality of manufacturing of component parts and there is a large number of thin-walled fashioned component parts which negatively affects reliability of prior art air hammer apparatuses. This is the reason why the annular air hammer apparatuses have not come into a commercial use up to the present.
Known in the art is an annular air hammer apparatus for drilling wells (cf. USSR Inventor's Certificate No. 1133388, Int.Cl. E 21 C 3/24,1985), comprising a hollow cylindrical body in which an annular hammer is movably received and a stepped air distribution tube having inlet and exhaust ports is rigidly fixed, a pipe for receiving drillings and rock breaking tool movably mounted in the lower part of the body, having an axial passage for establishing communication of the interior space of the pipe for receiving drillings with the well bottom space, and blowing passages for supplying air to the pipe for receiving drillings. The air distribution tube cooperates with the hammer and defines therewith a work stroke chamber and defines a return stroke chamber with the body.
This design of the apparatus allows the impact power to be increased upon back pressure rise in the exhaust duct thereof which occurs when the pipe for receiving drillings is choked and also when the well comes deeper or water influx occurs. However, in the prior art apparatus, the main part of the exhaust air is admitted to the pipe for receiving drillings through the exhaust ports of the stepped air distribution tube which are spaced somewhat apart from the well bottom, and only a small fraction of air is fed to the well bottom through the blowing passages of the rock breaking tool. For that reason, the broken rock is slowly displaced through the pipe for receiving drillings up to the exhaust ports and only then will the velocity of its ascent increase under the action of the exhaust air flow. With such a system of air flow distribution, drillings plugs are very likely to form in the axial passage of the rock breaking tool and in the lower part of the pipe for receiving drillings. The broken rock coming at level with the exhaust ports may enter therethrough the working chamber of the impact mechanism to cause a rapid wear of friction surfaces and even jamming of the hammer. In addition, because of a change in air pressure in the exhaust duct which is caused by a change in its capacity, operation of the impact mechanism becomes unstable as regards both blow rate and impact energy. These factors, considered in combination, lower reliability of the prior art apparatus in operation.
It is an object of the invention to improve stability in operation and level of output parameters of the apparatus. Another object of the invention is to improve reliability of the apparatus in operation.